Today, digital cellular radio is rapidly displacing analog cellular phone systems as the cellular communication of choice throughout the world. For example, the European digital cellular radio system, GSM (Global System for Mobile communications) is currently in wide use in over 50 countries. The digital cellular radio system like GSM is a fully digital system based on a time division multiple access (TDMA) system.
The GSM allows transmission of up to 8 different messages in one channel. Further, the GSM system transmits and receives data in the form of frames at different times. The transmission and reception of data frames usually utilize antennas. Hence, GSM requires a switching circuit to switch to and from transmission mode to reception mode.
The TDMA systems such as GSM utilize portable cellular telephone devices (i.e., handsets) to communicate. These wireless mobile communications devices such as cellular phones allow people to overcome the barrier of distance and the requirement of wires to communicate wherever they may be located. In recent years, due in large part to the design of smaller and more efficient power supply circuits, wireless mobile communications devices have evolved to fit into the palm of a hand.
Most of the power supply circuits in a typical wireless handset operate from a positive voltage power supply, with the system battery's negative terminal connected to the ground plane of the handset. However, some circuits often require a negative voltage power supply without using a second battery. For instance, PIN diode switches require a strong reverse bias from a negative power supply voltage to achieve adequate radio frequency (RF) isolation when turned off. Also, GaAs MESFET and other depletion FETs usually require negative gate bias for proper operation. In both of these instances, the current drawn from the negative supply is usually much smaller than 1 mA (milliampere).
One of the conventional approaches for developing a voltage of polarity opposite of the system battery employs a switching type of DC-DC converter. The DC-DC converter efficiently generates an AC waveform by switching the DC input at some predetermined frequency. Then the AC voltage swing is stepped up or down as needed, and then finally rectified and filtered. If a negative voltage is desired, the polarity of rectification is set accordingly. The application of the switching DC-DC converter is not limited to negative outputs. In many instances, the switching DC-DC converter is used to derive a positive voltage that is greater than the battery input. The DC-DC converter technology is highly developed and integrated chips (ICs) performing these functions are widely available in the market.
Unfortunately, for the wireless handset, and for radio applications in general, the disadvantage of the switching DC-DC converter is the unwanted emission of its switching frequency and harmonics, which cause interference with the radio circuits. That is, power supplies for these portable TDMA devices transmit harmonics and noise from their internal circuits to the rest of the TDMA device. In particular, the internal switching of the DC-DC converter produces harmonics that are transmitted to the rest of the power supply circuit to reach other sensitive parts of the wireless transceiver. In addition, switching between transmission and receive modes produces added noise that are transmitted to the rest of the power supply circuit and the transceiver. In this manner, noise severely degrades the reception and transmission quality of a TDMA device.
Furthermore, TDMA systems require switching between transmission and receive modes, and also at times, switching between two antennas. PN junction based PIN diodes have been widely used as switching devices for these switching modes due to their optimum characteristics in linearity and loss properties. Generally, one pin diode acts as a switch for a transmitter and another pin diode regulates a receiver. When one diode for the transmitter, for example, turns on, the other diode connected to the receiver, ideally, should have infinite impedance to prevent power from going into the receiver. In practice, a large signal voltage swing can begin to turn on a diode that is supposed to be off, allowing a portion of the signal to pass through when the signal is supposed to be blocked.
One design approach has addressed the forward bias problem of the diode by introducing a reverse voltage larger than the incoming input voltage swing on the PIN diode. This method typically uses a negative step-up voltage regulator. However, this method has been inefficient and costly to implement. First, the negative step-up voltage regulator is expensive to manufacture because it includes inductors, capacitors, etc. Second, since the voltage regulator is on all the time, the switching power voltage supply produces large harmonics resulting in undesirable transmission of noise.
Thus, what is needed is a circuit and method of providing a clean voltage power supply while minimizing transmission of noise and harmonics associated with AC signals and switching of transistors to other parts of a TDMA transceiver.